Fissionable fuel materials such as oxides of uranium, plutonium or thorium, and combinations thereof, are typically formed into small cylindrical slugs or pellets and housed within sealed tubes or elongated containers sometimes referred to in the art as "cladding". Large-capacity power-generating nuclear fission reactor plants normally have several hundred sealed cladding tubes for housing fissionable fuel. To facilitate periodic refueling, which commonly is performed by replacing fractional portions of the total fuel at intervals and rearranging other fractional portions, these fuel rods or pins are conventionally assembled into bundles or groups of elements which can be handled and manipulated as a single composite unit.
The fuel rods of each bundle are held mutually parallel and spaced apart by mechanical means. A typical fuel bundle comprises, e.g., an 8.times.8 or 9.times.9 array of spaced fuel rods. The cladding is usually more than 10 ft long, e.g., 14 ft, and approximately 1/2 inch in diameter, the cladding tubes being spaced from each other by a fraction of an inch. The spacing is required to permit an ample flow of heat-removing coolant, such as water, over the full exterior surface of the cladding for effective heat transfer and thus effective reactor operation.
To inhibit the fuel rods from bowing and vibrating due to high heat and high velocities of coolant flowing thereabout, which could cause adjacent fuel rods to contact and in any case could impede or unbalance coolant flow, the fuel rods are retained in a spaced-apart array or relation by means of a plurality of spacing grids (hereinafter "spacers") positioned at intervals along the fuel rod length.
A typical spacer comprises a plurality of parallel cells which are welded to each other and to a surrounding spacer band to form a lattice of cells. Each cell receives one fuel rod. Each fuel rod passes through a plurality, e.g., seven, of spacers. These spacers are mutually aligned and spaced along the length of the fuel rods. Each spacer receives a different axial portion of the plurality of fuel rods making up the fuel bundle. The spacers provide intermediate restraint and support transverse of the fuel rods, thereby preventing lateral bowing and vibration which could damage the fuel rods or impede effective coolant flow intermediate and around each fuel rod. Spacers for securing bundles of fuel rods often incorporate spring and stop members which press against the fuel rods in metal-to-metal contact as a means of securely gripping and holding the fuel rods in position. The fuel rods additionally have their ends supported in respective sockets of upper and lower tie plates.
The fuel rod bundle assembly is also typically surrounded by an open-ended tubular fuel channel of suitable cross section, e.g., square. The fuel channel directs the flow of coolant longitudinally along the surface of the fuel rods and guides the neutron-absorbing fission control rods which reciprocate longitudinally intermediate adjacent bundles.
Referring to FIG. 1, a typical nuclear fuel bundle 10 comprises a group of spaced-apart, mutually parallel fuel rods 12. Each fuel rod comprises a cylindrical container 14 (i.e., cladding) which houses a vertical stack of pins or slugs of fissionable fuel (not shown) sealed therein. Each fuel rod 12 is transversely secured in the parallel array by a series of spacers 16 positioned at intervals along the length of the fuel rods. The ends of the fuel rods of each bundle are fixed within respective sockets of upper and lower tie plates 18 and 20. The bundle assembly of grouped fuel rods 12 and spacers 16 is surrounded by an open-ended fuel channel 22.
FIG. 2 shows a top view of a typical spacer 16 comprising a plurality of parallel cells or ferrules 25 welded to each other and to a spacer band 27 to form a lattice or grid. Less than all of the ferrules 25 are shown for the sake of convenience.
As best seen in FIGS. 3A and 3B, each pair of adjacent spacer ferrules 25 and 25' share a common spacer spring 26 which biases fuel rods 12 and 12' in opposite directions toward a respective set of stops 24. In the absence of intervening means for deflecting the spring, the spring will be deflected by contact with the fuel rod. The interference between spring 26 and the fuel rods 12 and 12' is best seen in FIG. 3B. The spring 26 exerts a force of several pounds on the surface of each fuel rod during contact.
The structural components of the spacers--especially the spacer springs--must be periodically inspected for defects, such as cracks, corrosion, erosion and separated weld joints. Historically, inspection of irradiated fuel bundle spacers has required the complete disassembly of a fuel bundle at a reactor site in order to retrieve the spacers. The spacers are loaded into a cask, which is certified for shipment of radioactive materials, and shipped to a radioactive materials laboratory for destructive inspection. The above process, aside from being expensive and time consuming, does not allow reinstallment of the spacers for additional exposure to the reactor environment.
A fiber-optic scope, which is specifically constructed to perform examinations of fuel bundle spacers, permits the examination of spacers at the reactor site without the need to disassemble or otherwise disturb the bundle. After the examination, the bundle remains in the reactor core for additional exposure.
It is essential that the spacer springs be inspected because in some cases spacer springs may fail. This raises the specter of pieces of spring breaking off and recirculating in the coolant. Such pieces may block the flow of coolant through a fuel bundle when lodged in a coolant inlet or may even cause damage to the recirculation pump which in turn might break up into pieces. To avoid such failure, it is essential that the spacer springs be inspected.
Conventional fiber-optic scopes of a first type are constructed to enter the fuel bundle from a side and to view the spacer components from above or below. Scopes of the side-entry type require cumbersome and expensive manipulators for locating the scope within the fuel bundle. Conventional fiber-optic scopes of a second type are designed to be inserted from above into a spacer ferrule having its fuel rod removed. Scopes of the first type comprise a flat array of optical guides which, although thin enough to fit between adjacent rows of fuel rods, cannot be inserted into a spacer ferrule.
A known fiber-optic scope of the side-entry variety comprises a flat array of optical guides. The distal end of that scope is constructed as shown in FIGS. 4A, 4B and 4C. Light is supplied to the spot to be inspected by light guides 30a and 30b; light reflected from the inspected spot is transmitted to an ocular lens (not shown) via an image guide 32. Each guide consists of a bundle of fused optical fibers terminating at a planar interface of a respective prism 34. Each prism reflects incoming light at its inclined back interface, preferably by an angle of 90.degree.. The flat array may consist of any number of guides arranged in a repeating pattern of two light guides followed by one image guide or any other suitable pattern. These known scopes require a cumbersome manipulator and cannot be inserted inside the spacer ferrules.